The Thaumic Duplicator is a piece of thaumaturgical machinery that can duplicate items.
Before you are able to craft the Thaumic Duplicator, you need to research it with a Quaesitum first. As the duplicator belongs to the Lost Knowledge research tree, you should be able to get the theory for it easily.
The duplication of items comes, however, with quite a high cost: It costs five times more vis to create an item than you would receive from burning it in a Crucible - see Thaumcraft Math and Vis Conversion for detailed tables. While not a cheap process, copying is still worthwhile if you need to get those rather hard to find items (such as melon seeds, blaze rods or ender pearls).
The duplicator receives Vis from conduits or an adjacent crucible. The duplicator, however, must be connected to conduits one of it's three input sides, unlike other Thaumic devices. These sides are the 'back', 'left', and 'right' sides of the device when facing the naturally placed 'front' side. It will not take input from the bottom.
You can repeatedly copy the same item by toggling the arrow button below the progress bar in the GUI. Some objects, however, cannot be copied (for example Vis crystals!). The duplicator can only copy simple items or those designated as "common" within Minecraft. Those are usually items with white tooltip text. Most Thaumcraft items are magical by nature (which you can discern by the blue or yellow tooltip text) and thus cannot be copied. Items like the Brain-In-A-Jar cannot be copied.
Like all other Thaumcraft machines, duplicators can be deactivated with a redstone signal.